Abstract
In the present study, adsorption of Cr(VI) ions from aqueous solutions was investigated and compared under various conditions using two different nano-structured adsorbents, i.e., carbon nanotubes (CNTs) and low-cost activated carbons (AC). Walnut shell, an agricultural solid waste, was used as a raw material for the preparation of ACs. Multi-walled CNTs (MWCNTs) with average diameter of 20 nm and length of about 2 mm were also used for comparison purposes. Adsorption studies were carried out by varying the parameters such as: treatment time, metal ion concentration, adsorbent amount and pH. The adsorption capacities of AC and MWCNT for Cr(VI) ions were measured as 35 and 24 mg g−1, respectively. The efficiency was observed fairly high at pH = 2–3 for AC and pH = 5–6 for MWCNTs. The adsorption was significantly enhanced by increasing the adsorbent dose up to 0.4 and 0.3 g for AC and MWCNTs, respectively. It was also determined that Cr(VI) adsorption behavior follows both Langmuir and Freundlich isotherms. The content of functional groups, which was obtained by applying the Boehm’s method, revealed that phenolic groups are mostly present on the surface of MWCNTs, while basic groups are predominant on the walnut shell AC structures.
Highlights
Heavy metals are often present in the aquatic streams through different industrial wastewaters
The content of functional groups, which was obtained by applying the Boehm’s method, revealed that phenolic groups are mostly present on the surface of Multi-walled CNTs (MWCNTs), while basic groups are predominant on the walnut shell activated carbons (AC) structures
Chemical and thermal treatment processing could affect the adsorption capability of adsorbents for the removal of chromium ions as these treatments result in some functional groups on the solid surface [38]
Summary
Heavy metals are often present in the aquatic streams through different industrial wastewaters. Due to their toxicological importance in the ecosystem, agriculture and human health, pollution by heavy metals has received wide spread attention [1, 2]. One of the extremely toxic heavy metals affecting the environment, is present in the waste water as a result of its industrial application such as tanning, metallurgy, plating and metal finishing. Once chromium is introduced into the environment, it exists in two stable oxidation states, i.e., Cr(III) and Cr(VI) [3]. The trivalent form is relatively innocuous, but the hexavalent chromium is very toxic, highly mobile in soil and aquatic system, and is a strong oxidant capable of being adsorbed by the skin [4]. The tolerance limit of Cr(VI) for discharge into the inland surface water is set to 0.05 mg L-1 [5]
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